The cooling of an electronic equipment unit in a housing is normally obtained by removing the calories produced inside the housing toward the external environment by means of an air stream, possibly pulsed, penetrating into the housing through ventilation orifices situated in the bottom part and serving as nozzles, coming into contact with the components of the electronic equipment and being removed in the top part of the housing through ventilation orifices serving as vents. The calorie removal capabilities increase with the cooling air flow rate. In the case of the aeronautical standards such as Arinc 600 notably, the diameter and the number of the ventilation orifices are constrained so as to observe a head loss for a standardized air flow rate, which is itself a function of the dissipated power. This constraint makes it possible to distribute the pulsed air stream between the various computers.
When an electronic equipment housing also serves as electromagnetic shielding, which is almost always the case for equipment on board an aircraft, the effectiveness of the electromagnetic shielding also imposes a reduced diameter on the ventilation holes.
These limitations on the diameter and on the number of the ventilation holes greatly restrict the cooling capabilities by natural convection and very often require the usable temperature range to be limited in the event of loss of the forced ventilation.
The electronic equipment units designed to be installed on board aircraft are usually placed in housings provided with ventilation orifices of a diameter and of a quantity that are insufficient to allow normal cooling solely by natural convection. These housings are placed on a pulsed air distribution box, a kind of organ windchest, distributing to them, through their ventilation orifices, a stream of pressurized cooling air conforming to precise specifications, for example those given in the ARINC 600 standard relating to the configuration of the housings and subracks used in aircraft to house replaceable electronic equipment units that are also said to be “rackable”.
Such an arrangement poses the problem, critical when it comes to safety, of the necessary continuity of certain functions handled by the electronic equipment units in the event of loss of the pulsed cooling air stream.
Should the pulsed air stream be lost, the cooling is then provided only by a natural convection that is not very effective because of the excessively small useful section of the ventilation orifices that are limited in diameter and in quantity: in diameter by the requirements of the electromagnetic shielding and in quantity by the head loss imposed by the standard, and because of the external air volume available under the housings that is reduced through the content of the pulsed air distribution box. The temperature of the equipment units then increases significantly, which degrades their temperature operating margin.
Control of the operating temperature of an electronic equipment unit in the event of loss of the pulsed air stream is the main limitation encountered when seeking to reduce its bulk and to increase its functionalities by increasing the density of the electronic circuitry, because both are always accompanied by an increase in the production of calories by the liter.
To improve the cooling of an electronic equipment housing with cooling by pulsed air in the event of loss of the forced ventilation and allow for an increase in the compactness and the integration density of the electronic equipment, the applicant has already proposed, in a French patent application filed on Aug. 30, 2006 under the number 06 07628, providing an electronic equipment housing with orifices for ventilation by natural convection on its bottom and top walls and internal manifolds for distributing pulsed air fed with air under pressure via a distribution box placed at the back and connected to a pressurized air circulation duct.